Casting material for producing casting molds for casting high-melting point materials

ABSTRACT

The casting-mold material designed to manufacture casting molds used in casting high melting-point substances. The casting mold formed from a powder component containing magnesium oxide and aluminum oxide and a liquid component. The powder component additionally contains a calcium aluminate and an alkali fluoride or an earth alkali fluoride and the liquid component contains a polyelectrolyte and a basic component, the latter determining the liquid component pH value between 10 and 14. A fluid slip is prepared by mixing the powder and liquid components and is dried, hardened and sintered to produce a hollow mold.

The present invention relates to a casting-mold material used tomanufacture casting molds for casting high-melting point substances asdefined in the preamble of claim 1.

This casting-mold material is composed of a powder component and of aliquid component which are stirred into a fluid slip that, followingdrying and hardening, shall be manufactured by sintering into a hollowmold. The ceramic mold so made is filled With a molten metal, preferablymolten titanium or titanium alloys. Upon removing the mold, the castobjects will offer good details.

The casting mold material is appropriate for casting titanium andtitanium alloys, in particular for dental applications to produceprecisely fitted dental crowns, bridges or model casts offeringexcellent surface qualities and free of gas inclusions. Because themolten substance will shrink after being poured into the casting moldand then will solidify, the ceramic hollow mold must be made to expandcommensurately following sintering. This compensation is implemented byone or several expansible components in the casting mold material.

When being cooled, titanium and titanium alloys shrink substantiallyless then a conventional dental alloys such as of gold/cobalt orchromium/cobalt. As regards the known encapsulating substances beingused, compensation of expansion is carried out by adding ammoniumdihydrogen phosphate, which reacts with magnesium oxide to form coarseMgNH₄PO₄×6H₂O and by changing the phase of silicon dioxide.Advantageously quartzes and phosphates shall not be used because theyreact with the titanium melt.

The objective of the present invention is to create a casting-moldmaterial which is more fluid, more universally applicable and less timeconsuming and more economical than the casting-mold materials of theknown state of the art.

The invention attains the above objective using a casting-mold materialexhibiting the features of claim 1.

The casting-mold material of the present invention attains expansion byforming spinel above approximately 850° C. in the preheat oven. Comparedto the state of the art, the casting-mold material exhibits far betterfluidity, higher green hardness and more rapid setting. It also offersthe advantage of being suitable not only for casting dental crowns andbridges, but also as a universal, encapsulating substance for speed andmodel casting.

The improved fluidity of the casting-mold material of the presentinvention is mainly attained by means of the polyelectrolyte. The highergreen mechanical strength and hence the feasibility of model casting isattained by the calcium aluminate. The setting rate is determined inpart by the calcium aluminate and by the ingredients of the liquidcomponent. The zirconium oxide implements the improved resistance tothermal shocks.

In a preferred embodiment mode of the present invention, the powdercomponent of the casting-mold material additionally contains spinel,appropriately in a proportion by weight of 0.1 to 20.0%, preferably 1 to10%. The spinel admixture allows controlling the expansion.Appropriately the spinel exhibits a median grain size d₅₀ of 3 to 20microns, preferably 8 to 12 microns.

Appropriately the powder component contains by weight 40 to 60%,preferably 45 to 55% magnesium oxide, also 15 to 30%, preferably 21 to29% aluminum oxide. Moreover the powder component may appropriatelycontain by weight 0.5 to 20.0%, preferably 1 to 10% zirconium oxide.Increased mechanical strength is attained thereby following sintering,furthermore increased hardness and higher thermal resistance.

Appropriately the calcium aluminate in the powder component should bepresent in proportions by weight of 5 to 30%, preferably 10 to 20%. Thisproportion assures improved green mechanical strength and improvedcontrol of material setting.

Lastly the powder component may appropriately contain lithium fluoridein a proportion by weight of 0.5 to 20.0%, preferably 1 to 10%, toattain higher green mechanical strength.

The polyelectrolyte contained in the liquid component may be apolycarboxylic acid, preferably a polyacrylic acid or a polymethacrylicacid or one of their salts. Said polyelectrolyte appropriately should bepresent in a proportion by weight of 0.2 to 10.0%, preferably 0.5–3.0%.

Moreover the liquid component appropriately should contain a basiccomponent, preferably an alkali hydroxide (for instance lithiumhydroxide) in a proportion by weight of 0.5 to 10.0%, preferably 1 to9%.

A preferred embodiment mode of the present invention makes use ofmagnesium oxide in coarse and fine form, said coarse form exhibiting amedian grain size d₅₀ of 100 to 170 microns, preferably 110 to 140microns and the fine form exhibiting a median grain size d₅₀ less than10 microns, preferably 3 to 7 microns. The fine magnesium oxide causesaccelerated setting and the coarse magnesium oxide causes improvedresistance to thermal shock.

The weight ratio of coarse to fine magnesium oxide appropriately shallbe between 9:1 and 5:1, preferably 8:1 to 6:1.

The median grain size d₅₀ of the aluminum oxide used in the powdercomponent of the present invention shall be between 3 and 15 microns,preferably between 5 and 10 microns.

The zirconium oxide used preferentially in the powder componentappropriately shall exhibit a median grain size d₅₀ between 3 and 15microns, preferably between 5 and 10 microns.

The calcium aluminate used in the powder component appropriately shallexhibit a median grain size d₅₀ of 5 to 20 microns, preferably a mediangrain size d₅₀ between 8 and 12 microns.

Appropriately the liquid component is water, preferably de-ionized ordistilled water, optionally containing an added alcohol having a boilingpoint less than 100° C., preferably ethanol, 1-propanol, 2-propanol ortertiary butyl alcohol. Because the alcohol evaporates more easily thanwater, better drying is attained.

The invention and its further developments are elucidated below byseveral illustrative embodiment modes.

EMBODIMENT MODE 1

Powder component Liquid component (all data in % by wt) (all data in %by wt) Magnesium oxide 51.5 Lithium hydroxide 2.0 Aluminum oxide 25.0Polyacrylic acid sodium salt 2.0 Zirconium oxide 4.0 Water 96.0 Calciumaluminate 14.5 100.0 Lithium fluoride 7.0 Spinel 1.0 100.0

The mixing rate is 100 g powder component to 18 ml liquid component.Mixing time is 60 seconds. Setting time is 45 minutes.

EMBODIMENT MODE 2

Powder component Liquid component (all data in % by wt) (all data in %by wt) Magnesium oxide 55.5 Lithium hydroxide 1.0 Aluminum oxide 21.0Polyacrylic acid sodium salt 0.7 Zirconium oxide 3.0 Isopropanol 4.0Calcium aluminate 13.0 Water 94.3 Lithium fluoride 5.5 100.0 Spinel 2.0100.0

The mixing rate is 200 g powder component to 37 ml liquid component.Mixing time is 45 seconds and setting time is 40 minutes.

EMBODIMENT MODE 3

Powder component Liquid component (all data in % by wt) (all data in %by wt) Magnesium oxide 52.0 Lithium hydroxide 1.0 Aluminum oxide 23.0Polymethacrylic acid 1.5 Zirconium oxide 6.0 Isopropanol 3.0 Calciumaluminate 11.5 Water 94.5 Lithium fluoride 7.0 100.0 Spinel 0.5 100.0

The mixing rate is 200 g powder component to 37 ml liquid component.Mixing time is 45 seconds and setting time is 40 minutes.

EMBODIMENT MODE 4

Powder component Liquid component (all data in % by wt) (all data in %by wt) Magnesium oxide 50.0 Lithium hydroxide 1.0 Aluminum oxide 25.0Polyacrylic acid Na salt 3.0 Zirconium oxide 5.0 Water 96.0 Calciumaluminate 12.0 100.0 Lithium fluoride 4.0 Spinel 4.0 100.0

The mixing rate is 200 g powder component to 37 ml liquid component.Mixing time is 60 seconds and setting time is 40 minutes.

To manufacture the casting-mold material of the present invention, thepowder component and the liquid component are mixed or kneaded togetherin the manner of one of the above Embodiment Modes 1 through 4. Theliquid component contains a polyelectrolyte assuming that in the firstplace the casting-mold material shall be adequately fluid. Accordinglystirring provides a fluid, castable mass which shall be poured aroundthe master model of the cast body, said master model consisting of waxor a plastic burning without leaving a residue. Solidification of theliquid mass in part takes place by the calcium aluminate settinghydraulically. The setting rate of the liquid mass may be adjusted bythe quantity of calcium aluminate and magnesium oxide as well as by thecomposition of the liquid component in such manner that said mass may beprocessed conveniently while a very short setting time may still beattained.

Following being poured into a warm and vibration-free space, the mixedmass is allowed to set for at least 45 minutes. After about 45 minutes,the mass that has set can be placed into an appropriate preheating ovenand the oven program sintering the encapsulating substance may bestarted. This stage requires about 6 h, and during that time waxes,plastics and water will be expelled, the spinel expansion then ensuing.

Illustratively the oven program comprises a heating stage at atemperature rate of 4° K/minute until a temperature of 300° C. has beenreached, further a 30-minute dwell time at 300° C., another heatingstage at the rate of 7° K/minute until reaching a temperature of 900° C.and a cooling stage down to 450° C. Thereupon the sintered hollow moldis filled with molten titanium at a casting temperature of 450° C. in avacuum pressurized casting mold under an inert gas.

1. A casting-mold material to manufacture casting molds for casting highmelting-point substances, said material comprising: (A) a powdercomponent comprising magnesium oxide and aluminum oxide, and (B) aliquid component, wherein (C) the powder component further comprises acalcium aluminate and an alkali fluoride or an earth alkali fluoride,and (D) the liquid component comprises a polyelectrolyte and a basiccomponent, said basic component determining the pH value of the liquidcomponent between 10 and
 14. 2. The casting-mold material as claimed inclaim 1, wherein the powder component further comprises zirconium oxide.3. The casting-mold material as claimed in claim 1, wherein the basiccomponent includes an alkali hydroxide.
 4. The casting-mold material asclaimed in claim 1, wherein the powder component further comprisesspinel.
 5. The casting-mold material as claimed in claim 1, wherein thealkali fluoride is selected from the group consisting of lithiumfluoride and sodium fluoride.
 6. The casting-mold material as claimed inclaim 1, wherein the basic component determines the pH value between 11and
 13. 7. The casting-mold material as claimed in claim 2, wherein thepowder component contains a proportion by weight of zirconium oxide of0.5 to 20%.
 8. The casting-mold material as claimed in claim 1, whereinthe powder component contains a proportion by weight of calciumaluminate of 5.0 to 30.0%.
 9. The casting-mold material as claimed inclaim 1, wherein the powder component contains a proportion by weight ofone of alkali fluoride and earth alkali fluoride of 0.5 to 20%.
 10. Thecasting-mold material as claimed in claim 1, wherein the powdercomponent contains a proportion by weight of spinel of 0.1 to 20.0%. 11.The casting-mold material as claimed in claim 1, wherein the liquidcomponent contains a proportion by weight of a polyelectrolyte of 0.2 to10.0%.
 12. The casting-mold material as claimed in claim 1, wherein theliquid component contains a proportion by weight of the basic component,which is an alkali hydroxide, of 0.5 to 10.0%.
 13. The casting-moldmaterial as claimed in claim 1, wherein the polyelectrolyte contains apolycarboxylic acid.
 14. The casting-mold material as claimed in claim1, wherein the magnesium oxide is present in coarse form and in fineform, a median grain size d₅₀ of the coarse form being 100 to 170microns, and the median grain size d₅₀ of the fine form being less than10 microns.
 15. The casting-mold material as claimed in claim 14,wherein the magnesium oxide is present in a ratio by weight of coarseform to fine form of 9:1 to 5:1.
 16. The casting-mold material asclaimed in claim 1, wherein the aluminum oxide exhibits a median grainsize d₅₀ of 3 to 15 microns.
 17. The casting-mold material as claimed inclaim 2, wherein a median grain size d₅₀ of the zirconium oxide is 3 to15 microns, preferably 5 to 10 microns.
 18. The casting-mold material asclaimed in claim 1, wherein a median grain size d50 of the calciumaluminate is 5 to 20 microns.
 19. The casting-mold material as claimedin claim 1, wherein a median grain size d₅₀ of the spinel is 3 to 20microns.
 20. The casting-mold material as claimed in claim 1, whereinthe liquid component contains water, said water being selected from thegroup consisting of de-ionized water and distilled water.
 21. Thecasting-mold material as claimed in claim 1, wherein the liquidcomponent comprises an alcohol having a boiling point lower than 100°C., said alcohol being selected from the group consisting of ethanol,1-propanol, 2-propanol, and tertiary butyl alcohol.
 22. A fluid slip,prepared by mixing the powder component with the liquid component of thecasting-mold material claimed in claim
 1. 23. A method for castingtitanium and titanium alloys comprising the steps of: providing acasting mold manufactured from a casting mold material comprising: (A) apowder component comprising magnesium oxide and aluminum oxide, and (B)a liguid component, wherein (C) the powder component further comprises acalcium aluminate and an alkali fluoride or an earth alkali fluoride,and (D) the liguid component comprises a polyelectrolyte and a basiccomponent, said basic component determining the pH value of the liguidcomnonent between 10 and 14; and pouring molten titanium or titaniumalloy into said casting mold.
 24. The casting-mold material as claimedin claim 1, wherein the basic component includes lithium hydroxide. 25.The casting-mold material as claimed in claim 2, wherein the powdercomponent contains a proportion by weight of zirconium oxide of 1 to10%.
 26. The casting-mold material as claimed in claim 1, wherein thepowder component contains a proportion by weight of calcium aluminate of20.0 to 20.0%.
 27. The casting-mold material as claimed in claim 1,wherein the powder component contains a proportion by weight of one ofalkali fluoride and earth alkali fluoride of 1 to 10%.
 28. Thecasting-mold material as claimed in claim 1, wherein the powdercomponent contains a proportion by weight of spinel of 1 to 10.0%. 29.The casting-mold material as claimed in claim 1, wherein the liquidcomponent contains a proportion by weight a polyelectrolyte of 0.5 to3.0%.
 30. The casting-mold material as claimed in claim 1, wherein theliquid component contains a proportion by weight of the basic component,which is an alkali hydroxide, of 1 to 9%.
 31. The casting-mold materialas claimed in claim 13, wherein the polycarboxylic acid is selected fromthe group consisting of polyacrylic acid, polymethacrylic acid, and oneof their salts.
 32. The casting-mold material as claimed in claim 1,wherein the magnesium oxide is present in coarse form and in fine form,a median grain size d₅₀ of the coarse form being 100 to 140 microns, andthe median grain size d₅₀ of the fine form being between about 3 to 7microns.
 33. The casting-mold material as claimed in claim 32, whereinthe magnesium oxide is present in a ratio by weight of coarse form tofine form of 8:1 to 6:1.
 34. The casting-mold material as claimed inclaim 1, wherein the aluminum oxide exhibits a median grain size d₅₀ of5 to 10 microns.
 35. The casting-mold material as claimed in claim 2,wherein a median grain size d₅₀ of the zirconium oxide is 5 to 10microns.
 36. The casting-mold material as claimed in claim 1, wherein amedian grain size d₅₀ of the calcium aluminate 8 to 12 microns.
 37. Thecasting-mold material as claimed in claim 1, wherein a median grain sized₅₀ of the spinel 8 to 12 microns.